Page 531 - Small Animal Clinical Nutrition 5th Edition

Disorders of Lipid Metabolism 549

miniature schnauzers, are also at increased risk of clinical ill-
VetBooks.ir ness associated with hypertriglyceridemia characterized by
the inability to degrade chylomicrons. Though not definitive-
ly proven, familial traits are thought to cause these disorders.
Results of a limited survey of healthy adult dogs suggested
that primary hypercholesterolemia might occur within some
families of Doberman pinschers, rottweilers, a Shetland
sheepdogs (Sato et al, 2000), rough collies (Jeusette et al,
2004) and briards (Watson et al, 1993).
Secondary risk factors (i.e., particularly endocrine disorders,
certain drugs and possibly certain diets leading to hyperlipi-
demia) are known to occur but have not been well studied. For
example, profound fasting hypertriglyceridemia occurs incon-
sistently in dogs with unregulated diabetes mellitus. Clinical
signs associated with excess triglyceride concentrations typical-
ly include vomiting, diarrhea and abdominal discomfort. Ap-
proximately 30% of untreated hypothyroid dogs and from 25 to
30% of untreated dogs with pituitary-dependent hyperadreno-
corticism have excess serum cholesterol concentrations.
However, the relationship between clinical signs, if any, and the
hyperlipidemia has not been established.
Obesity is known to cause abnormalities of lipid metabolism
in people. Experimentally induced chronic obesity in otherwise
Figure 28-5. The positive chylomicron test. The lactescent serum
normal dogs fed a complete and balanced maintenance food
in both tubes was obtained from a dog with hypertriglyceridemia.
resulted in significantly higher concentrations of cholesterol in
The sample on the left is the serum immediately after separation
total plasma (+41%) and in VLDL (+125%), HDL (+45%) and
from the red blood cells whereas the sample on the right was
low-density lipoprotein (LDL) (+58%) fractions and signifi- allowed to stand undisturbed for 10 hours. The so-called “cream
cantly higher concentrations of triglycerides in total plasma layer” is comprised of triglyceride-rich chylomicrons. (Reprinted
(+75%) and in the VLDL (+118%). When switched to a low- with permission from Ford RB. Canine hyperlipidemia. In: Ettinger
energy, high-fiber diet that resulted in overall decreased energy SJ, Feldman EC, eds. Textbook of Veterinary Internal Medicine, 4th
intake, plasma lipid values decreased (Jeusette et al, 2005). ed. Philadelphia, PA: WB Saunders Co, 1995; 1417.)
In some animals, drugs are known to either decrease
lipoprotein degradation or increase lipoprotein production,
thereby causing hyperlipidemia. For example, dogs receiving
long-term phenobarbital therapy for regulation of idiopath-
ic epilepsy may develop hypercholesterolemia. The clinical
significance is unknown and may, in fact, be related to thy-
roid-hormone production or activity. Cats receiving mege-
strol acetate may secondarily develop diabetes mellitus,
which may culminate in altered lipoprotein lipase activity
and hyperchylomicronemia.

Etiopathogenesis
Normal Lipid Metabolism
LIPOPROTEINS
Cholesterol and triglycerides are hydrophobic molecules;
therefore, they cannot circulate in the aqueous milieu of blood
without being incorporated into complex, spherical macromol-
ecules called lipoproteins (Brown and Goldstein, 1987;
Chapman, 1980; Schaefer and Levy, 1985; Weinberg, 1987; Figure 28-6. Diagram illustrating the composition and structure of
Watson and Barrie, 1993). The water-soluble outer coat of the lipoproteins. Cholesterol and triglycerides comprise much of the
lipoprotein is comprised of phospholipids, nonesterified (free) lipoprotein core and are present in varying proportions. (Adapted
cholesterol and several unique proteins called apolipoproteins from Brody T, ed. Nutritional Biochemistry. New York, NY: Academic
(Figure 28-6). Cholesterol, in the form of cholesterol esters, Press Inc, 1994; 276.)
and triglycerides are carried within the nonpolar core of spher-
ical lipoprotein macromolecules. Abnormally high concentra-